Chromatography-based separation or fractionation methods are most often implemented in a separation system comprising (FIG. 1) a series of columns or column fractions interconnected in series, forming a closed loop. A porous solid of predetermined grain size constitutes the stationary phase. The mixture to be separated is fed into the column, then displaced by means of a carrier fluid or desorbent (EL) and the various constituents flow out successively according to whether they are retained more or less greatly by the stationary phase. Injection points for the mixture or feed F containing all of the constituents to be separated and the solvent or desorbent EL, and extraction points for an extract Ex containing the product to be upgraded, diluted in solvent, and for a raffinate Rf containing all the other constituents are distributed along this loop. These points delimit various zones (Z1 to Z4 for example). An identical liquid stream flows through all the columns or column fractions of a zone. A pump P is arranged somewhere in the loop to provide circulation of the fluid in the direction shown in the diagram.
In a real countercurrent separation system, a fixed and constant concentration profile develops where the positions of the injection and extraction points remain fixed. Adsorbent solid 3 and liquid 2 move in a countercurrent flow. A solid entrainment system and recycle pump P, both arranged at the junction of zones Z1 and Z4, respectively allow to send back the solid from the base to the top and, conversely, the liquid from the top to the base.
Systems known as simulated moving bed systems allow to overcome a major difficulty inherent in true moving bed methods, which consists in properly circulating the solid phase without creating attrition and without considerably increasing the bed porosity in relation to the porosity of a fixed bed. To simulate its displacement, the solid is placed in a certain number n of fixed beds (generally 4≦n≦24) arranged in series and it is the concentration profile that is displaced at a substantially uniform velocity all around a closed loop. In practice, successive switching of the injection and extraction points is obtained by means of a rotary valve or more simply of a series of suitably controlled on-off valves. This circular switching, carried out at each period, of the different incoming-outgoing liquid flows in a given direction amounts to simulating displacement of the solid adsorbent in the opposite direction.
The separation systems used for xylenes separation most often consist of four main zones. There are also systems with five zones where part of the extract separated from the solvent is reinjected between extract draw-off and feed injection. Others can also have five to seven zones where secondary fluids allow to rinse lines carrying successively several fluids so as to prevent contaminations.
In the text hereunder, the following variables are defined as:                controlled variables: variables that have to be constantly close to a previously determined set value and which show the smooth running of the process. It can be, for example, the purity of the constituents of an extract, the yield of the separation unit for a given constituent, etc.        operating variables: variables that can be modified by the operator, such as the flow rates or the valve switch period allowing to simulate displacement of the beds, etc.        control variables: variables that act mainly on a single zone, for example on the part of the concentration profile contained in a zone. These control variables are determined by the control algorithm and are translated into operating variables.        
It can be reminded that the goal of an advanced control system applied to a process is to calculate a control law (all of the values of the operating variables in time) so as to:                control operation, i.e. calculate a control law that can ensure the transition between two distinct values of one or more a priori selected controlled variables, and        regulate operation, i.e. calculate a control law allowing best compensation (in advance or at least asymptotically) of all the outside disturbances acting on the process so that the a priori selected controlled variables keep a quasi-constant value.        
In the case of a simulated countercurrent separation unit, regulation can also compensate for disturbances due to an evolution with time of the thermodynamic and geometric parameters of the adsorbent (of course for a limited deterioration of the adsorbent properties).
These objectives are reached with the automatic control process based on either a “black box” type technique, or on a more controlled approach allowed by non-linear modelling of the separation process.
Patent EP-875,268 (U.S. Pat. No. 5,902,486) filed by the applicant describes a method intended for automatic control of a simulated moving bed separation system for constituents of a mixture of circulating fluids, notably aromatic hydrocarbons, which can have notable flow rate or feed quality variations. Control of the process (of non-linear multivariable type carried out from a knowledge or linear model in the neighbourhood of a given working point, performed from input/output representation models) is carried out with a certain number of variable measurements at a plurality of measuring points along the loop (concentrations and flow rates for example) and of characteristic measurements of the fluids injected and extracted. Ratios respectively indicative of the ratio, in each zone, between the fluid flow rates and the simulated adsorbent substance flow rates are determined from current controlled variable values (constituents purity, yield of the system, etc.) depending on the measured variables. Values to be given to the operating variables to bring or bring back the controlled variables to determined set values are determined from these ratios. If four independent control variables are available for example, the four ratios in each zone, four controlled variables have to be determined.
The control process comprises a calculating algorithm which determines the ratios from the measurements obtained, which are necessary for calculation of the controlled variables. This calculation can be carried out in two completely different ways: either using a non-linear physical model of the true moving bed separation unit, or using a combination of monovariable linear models, each representing the behaviour of an output (a controlled variable) in relation to an input (a control variable), knowing that combination of these linear models is often referred to as “black box” by specialists. Determination of these simple models is performed from a set of experimental measurements obtained on the process working in a state close to its planned stable state.
In its developed xylenes separation version, the process is used to purify the paraxylene present in feeds containing mostly xylenes, but also C9 aromatics and paraffins in limited amounts. It is available in two versions: the standalone version, which allows to reach a purity above 99.80%, and the hybrid version which is dimensioned to reach a purity of the order of 95.00%. The latter version of the process, described for example in patent EP-531,191, is marketed with addition of a crystallization process allowing to reach the desired high purity. Units working in hybrid mode consist of at least 12 columns, whereas there are at least 24 columns for the standalone mode.
Whether a non-linear automatic control process or a black box type process, the goal is to calculate, from measurement of the concentrations of certain constituents necessary for calculation of controlled variables, ratios (Rk) respectively indicative of the ratio, in each zone, between fluid flow rates (Qk) and the simulated flow rate of adsorbent material (Qs) so as to bring or to bring back the controlled variables to determined set values. In a second stage, the values of the ratios thus determined will be converted to operating variables applied to the process by means of conversion formulas.
The control process thus allows the separation unit to be brought to a working point where the following four parameters are brought to specified values:
1. The purity of the paraxylene in the extract defined as follows:
      Purity    =                  Px        extract                              Px          extract                +                  IMP                                                            ⁢            extract                                ,  where    Pxextract is the paraxylene concentration in the extract, and    Impextract represents all the impurities in the extract.
Determination of this controlled variable requires online measurement in the extract, on the one hand, of the paraxylene concentration and, on the other hand, of all of the other constituents;
2. The paraxylene yield of the unit defined as follows:
      Yield    =          1      -                                    Q            raffinate                    ⁢                      Px            raffinate                                                              Q              raffinate                        ⁢                          Px              raffinate                                +                                    Q              extract                        ⁢                          Px              extract                                            ,  where    Pxextract and Pxraffinate respectively represent the paraxylene concentration in the extract and in the raffinate, and    Qraffinate and Qextract respectively represent the raffinate and extract flow rates.
Determination of this controlled variable requires online measurement of the paraxylene concentration in the extract and in the raffinate, and measurement of the extract and raffinate flow rates.
3. The amount of ethylbenzene Ebextract in the extract.
Determination of this controlled variable requires the same online measurement as developed for point 1.
4. The amount of paraxylene Pxzone1 at a point of zone 1.
Determination of this controlled variable requires development of a specific measuring point in zone 1, i.e. between solvent injection and extract draw-off.
If the first two controlled variables clearly correspond to production objectives, the last two are directly linked with the object of the present invention, i.e. optimization of the unit operation.
Control of the separation unit requires concentration measurements at three distinct points of the loop. These measurements are carried out by means of chromatography or Raman spectrometry, as described in patent FR-2,699,917 (U.S. Pat. No. 5,569,808) filed by the applicant. Online calculation of the purity and of the yield and measurement of the amount of ethylbenzene in the extract requires two measurements which provide the concentrations of the different constituents in the extract and in the raffinate. The last output Pxzone1 requires a measuring point in zone Z1. The length of an analysis ranges between some seconds (Raman spectrometry) and 20 minutes (chromatography). Considering the response time of the unit (between 4 and 8 hours), the quality of the process control is not affected by the analysis time if it remains less than one hour.
Conversion of the control variables (ratios Rk) to “conventional” operating variables is always possible, apart from the real physical application constraints linked with the dimensioning of the process and its equipment, because there is a one to one relation between them, a necessary condition for making the separation system perfectly controllable.
It is well-known that operation of a simulated countercurrent separation system is nearly identical to that of a true moving bed system if, for the latter, the flows circulating countercurrent to the main liquid flow meet the equivalence relations described in the aforementioned patent EP-875,268.
The control variables (ratios Rk) are determined in relation to these equivalences as the dimensionless ratios between the main liquid flow rates in each zone and the solid flow rate which is constant in the whole separation unit:
      R    k    =                    Q        k                    Q        s              .  
Selection of these ratios follows from writing of the material balance equations of the model of a true moving bed separation unit in the stationary state in a column portion that is discretized. The number of ratios is equal to the number of zones that make up the unit, each zone being characterized by a main liquid flow rate distinct from the contiguous zones.